This invention relates to a feed system for an antenna and, more particularly, to a composite feed covering two octaves of bandwidth and providing a common phase center for radiations in each of a plurality of signal bands radiated by the feed.
Various communication systems employ more than one frequency band for electromagnetic signals radiated from a transmitting station to receiving station. An important example of such a communication system is a satellite communication system wherein various bands of signals are transmitted between a satellite above the earth (synchronous orbit) and ground stations on the earth. Three such bands of interest herein, including C band, X band, and Ku band, extend in total two octaves of the communication frequency spectrum. Within each of the bands, there is frequency space allocated for reception of signals at the satellite and for transmission of signals from the satellite. The C band itself extends over approximately an octave, operates at both linear and circular polarizations, and includes a receive sub-band in the range of 3.625-4.200 GHz and a transmit sub-band in the range of 5.850-6.425 GHz. The X band includes a receive sub-band in the range of 7.250-7.750 GHz (gigahertz), and a transmit sub-band for transmission from the satellite in the range of 7.900-8.400 GHz. The Ku band operates at both linear and circular polarizations, and includes a receive sub-band from 10.950 to 12.750 GHz, and a transmit sub-band of 14.000-14.500 GHz. Collectively, these frequency bands extend over approximately two octaves of the communications spectrum.
Historically, it has been the practice to provide separate antennas for transmission or reception on each of the bands because there is insufficient bandwidth on any one of the antenna systems or terminals to transmit more than one of the bands. In some cases, where bands are close together and, collectively, do not occupy an excessive amount of spectral space, it has been possible to share a plurality of bands on one antenna. However, basically, separate antennas have been employed for different portions of the spectrum. In particular, there is no adequate single-point antenna feed system which can cover plural octave bandwidths which includes C, X and Ku bands.
A problem arises in the case of satellite communication transportable earth stations in that there is a need for minimization of transportable payload weight. The use of numerous antennas for communication at various frequency bands defeats the purpose of minimization of payload weight. In addition, it is advantageous to employ a common phase center for all radiations transmitted from the earth station and received at the earth station. There is no common phase center in the situation wherein several antenna feeds are mounted at different times upon an earth terminal. It has been necessary to change the feed system for each frequency band and to refocus the feed, this requiring time and trained personnel. The same problem exists for an earth terminal at a fixed location because it is still necessary to perform the difficult and tedious process of exchanging feeds and refocusing.
The foregoing problem is compounded by the foregoing spectral utilization. The C band and the Ku band are commercial satellite bands which are spaced apart in the spectrum and, therefore, facilitate the filtering of signals in the two bands so as to permit transmission on one band without significant interference with signals on the other band. However, in the present situation, there is also need to employ the X band which is a military band in conjunction with the C band. In the present situation, it is contemplated that either one of the Ku and the X bands may be employed with the C band or, possibly, that both the Ku and the X bands may be employed concurrently with the C band. However, due to the fact that the X band is contiguous to the C band, it is difficult to separate the two bands in a common antenna system and, furthermore, presently available antenna and feed structures are unable to accomplish this task adequately.